This invention relates to a method of using waste materials to form a material usable as a fertilizer fill material or landfill cover. The invention relates to the combination of municipal sewage sludge and combustion residue such as that produced in coal burning electricity generating operations.
Fly ash is essentially fine solid noncombustible mineral residue typically resulting from coal-burning operations. It does not however include other more coarse combustion by-products such as bottom ash, cinders, or slag. Fly ash typically comprises very fine particles, usually containing silica (SiO.sub.2), alumina (Al.sub.2 O.sub.3), ferric oxide (Fe.sub.2 O.sub.3), calcium oxides (CaO), and small quantities of other oxides and alkalies. Fly ash is an artificial possolan and is generally not cementious in itself, but with the presence of water and lime compounds, it forms a cementious product. These lime compounds often exist naturally in the fly ash or can be supplied by the addition of a lime source such as cement or kiln dusts.
Fly ash by-product can be a problem for many coal burning industries, because the fly ash can be swept away by ordinary air currents, thereby polluting the air and ultimately settling in undesirable places. As a result, the coal-burning industry must be ever mindful of present and possible future environmental regulation, potential negative public opinion, and potential legal liability.
The methods of wasting fly ash which have traditionally been used are the "sluicing" method and the "dampening" method.
The "sluicing" method mixes fly ash and liquid to a liquid content of greater than 90% by weight. The mixture is then pumped into a holding pond where it is exposed to ambient conditions and allowed to dry until the next batch of fly ash and water is added.
The sluicing process has several disadvantages. First, in arid regions, large quantities of liquid are sometimes not readily available or only available at a very high cost. Second, the sluicing method often requires expensive site preparation, including the need for special cell liners and embankments. Third, the volume of water used in this method causes it to be very land consumptive, and the necessary land is not always available. Finally, a large holding pond of the end-product can be hazardous, because the cell liner or embankment could leak; also, wildlife or curious children could be attracted to the holding pond, which could result in any one of a number of serious consequences.
Due to the aforementioned disadvantages, the sluicing method of wasting fly ash is not well suited to the needs of today's coal-burning industry.
In the "dampening" method, just sufficient liquid is added to the dry fly ash to calm the dust, and this facilitates the handling of the fly ash with conventional earth moving equipment. Although the liquid content will vary with the type of fly ash, a liquid content of about 5% to about 25% by weight is typically used. Current dampening systems commonly mix a continuous flow of fly ash with a continuous flow of liquid. Although the liquid flow rate can be easily controlled, the fly ash flow rate often cannot; as a result, the liquid content of the dampened fly ash often varies by 50% or more. This lack of uniformity produces handling problems. At lower liquid concentrations, unwanted fugitive dust is often generated; at higher concentrations, the end product becomes a paste or sludge which makes handling more difficult. Dampening systems are therefore becoming less suitable for today's needs.
Due to the inadequacies of the sluicing and dampening methods, an innovative alternative method has been created and is disclosed in Pound, U.S. Pat. No. 4,313,762, "Method of Wasting Fly Ash and Product Produced Thereby" and Pound, U.S. Pat. No. 4,461,601, "Slurry System for Wasting Fly Ash Having Nonleachable Self-Sustaining End Product".
Briefly, the Pound patents disclose a fly ash disposal system where, after the fly ash cools and is removed from a coal-burning plant boiler, it is transported by conventional means to a closed storage area to await transportation to a site located some distance from the coal-burning facility. The bone dry fly ash is discharged from the power station storage area into a pneumatic bulk transport truck. After filling, the truck is sealed to prevent dust leakage during transit. The substantially dust free closed chambered transport vehicle then brings the fly ash to a processing plant storage unit. The fly ash is ultimately transferred from the storage unit into a slurrier where it is mixed with an amount of liquid. The resultant slurry is then discharged via a slurry pump to a disposal cell.
Sewage sludge is the end by-product of typical community waste treatment facilities. In typical sewage treatment facilities, the sludge is chemically conditioned and dewatered by vacuum filters. The dewatered sludge is then transferred to trailer dump trucks and hauled to a disposal site for further treatment or disposal.
Disposing of sewage sludge has been accomplished by a number of methods. In some waster disposal methods, the sludge is incinerated. However, incineration typically results in high energy costs. Furthermore, incineration disposal methods also produce gaseous by-products which may cause environmental concerns.
Other disposal processes involve the transporting of sludge to a disposal site in dump trucks. These trucks are then driven over refuse located at the disposal site. The sludge, including its contained water, is thereby dumped onto the refuse. The sludge is mixed with the refuse and then compacted with a bulldozer. In this way, the refuse is intended to soak up the liquid contained in the sludge, thereby reducing the environmental impact upon the surrounding region. The amount of sludge which can be mixed with refuse is typically controlled by regulatory agencies. Whether regulated or not, however, landfills can become saturated with waste, thereby generating environmental hazards and concerns.
Other methods of sewage disposal include burying the sewage sludge at a disposal site dedicated solely for such sludge. However, this method of disposal also raises environmental concerns due to ground water pollution, general instability and undesirable settling of the completed fill, and other hazards.
The effective and safe disposal of municipal sewage sludge is becoming more difficult. Locations where such material can be economically disposed are becoming more scarce. It has been found that methods previously thought safe are not completely effective in the long term. Sewage sludge contains large amounts of bacteria and other pathogens which must be destroyed or, at least, contained and prevented from spreading. The U.S. EPA has promulgated rules governing the type of processes that can be used to treat waste water sludge. These processes are directed at reducing pathogens at various levels.
Other prior art techniques for treating municipal sludge have included combining such sludge with fly ash and cement kiln dust, as suggested in U.S. Pat. Nos. 4,781,842; 4,554,002; 4,432,800; 4,341,562; 4,101,332; 4,038,095 and 4,018,617, or with fly ash and lime, as suggested in U.S. Pat. No. 4,028,130. However, depending upon their availability, kiln dust and lime can be prohibitively expensive to use.
It has been found that a material resulting from the use of a coal burning technique known as the fluidized bed process is usable as a replacement for lime. The fluidized bed process produces fluidized bed combustion residue ("FBCR") or bottom drain, which has been suggested for use, after wet pre-treatment (hydration) thereof, in combination with fly ash to stabilize scrubber sludge. In U.S. Pat. Nos. 4,250,134; 4,397,742; 4,344,796 and 4,397,801 (the "Minnick patents"), the inventor pretreats the FBCR with a process called "blunging", wherein water is added to the FBCR and the combination is mixed. The Minnick patents do not suggest the use of FBCR to treat municipal sludge.
In Nowicki et al, U.S. Pat. No. 4,472,198, "Process and System of Wasting Fly Ash And Product Produced Thereby", a method and system is disclosed for disposing of fly ash and liquid industrial waste. The product of this method is environmentally safe and comprises a mixture of dry fly ash and liquid additive having a liquid additive content of about 5%-25% by weight. The process relates to a mixing operation which closely controls the liquid additive to yield a uniformly conditioned product; this product can be subjected to earth handling equipment pressures almost immediately. The process also typically eliminates cell preparation and greatly reduces leaching at the disposal site.
However, the Nowicki process is not applicable to all forms of waste disposal. The disclosure of Nowicki is directed to liquid waste disposal which may not be suitable for sewage sludge disposal due to the sludge's non-aqueous physical properties.
The treatment of sewage sludge, as contrasted with the treatment of scrubber sludge, must address the serious problem of bacteria. Sewage sludge contains large amounts of bacteria which must be destroyed and prevented from regeneration. It may also contain amounts of undesirable "heavy" metals which must, in accordance with EPA regulations, be disposed of in a controlled manner. Generally, sewage sludge also contains a higher water-to-solids ratio than other industrial sludge.
On the other hand, sewage sludge contains many nutrients which make it a possible source of fertilizer. The handling and delivery of raw, de-watered sewage sludge is, however, difficult, because of odor, bacterial content, and other physical properties. Regulations concerning methods of handling and application can be found in Title 35 Subtitle C, "Land Applications of Municipal Sludge" from the EPA.
Also, the fact that sewage sludge is a result of human activity means that its source will necessarily be near densely populated areas. In contrast, scrubber sludge may be produced at power generating stations quite distant from populated areas, making disposal thereof less critical or less costly.
As a result, an object of this invention is to provide a fly ash and sewage sludge wasting method and system which is environmentally safe, economical and reliable.
It is an object of the present invention to treat municipal sewage sludge with FBCR, FBCR and fly ash, or fly ash alone for the purpose of solidification and/or detoxification.
Yet another object of the invention is to produce a fertilizer from municipal sewage sludge, which is easily stored, handled and delivered.
Still another object of the invention is to produce a material comprised of sewage sludge, an activator comprised of either FBCR, fly ash, or FBCR and fly ash, which is suitable for use as a landfill material.
Yet a further object of the invention is to produce a stabilized sewage sludge composition which is substantially free of pathogenic agents.
A further object of the invention is to provide a method of and system for wasting fly ash and sewage sludge whereby the fly ash is handled during transit and processed in a manner which is pollution free.
Yet another object is to provide a process and system of wasting fly ash and sewage sludge in which the end product will harden into a stable, environmentally acceptable mass.
Yet another object is to provide a fly ash and sewage sludge wasting system in which the final, hardened product meets all current environmental requirements, and is resistant to leaching and to percolation from ground water and rainfall whereby the pollution potential in the disposal area is greatly reduced.
Yet a further object is to provide a fly ash sewage sludge wasting system in which on-site dust at the disposal location is reduced to a minimum.
A further object is to provide a fly ash and sewage sludge wasting system and process which is more economical to operate than other conventional systems.
A still further object is to provide a sewage sludge and wasting system which is less land consumptive and more space efficient than previously known methods.
Other objects and features of the invention will become apparent to those skilled in the art from the following specification when read in conjunction with the annexed drawings.
These and other objects of the invention are obtained with a method by which sewage sludge, FBCR and fly ash are blended in a pug mill. The mixture is then stockpiled where it is allowed to cure for a minimum period of about 72 hours. During that period, the temperature of the mixture is maintained at a minimum level of about 100.degree. F. The alkalinity of the mixture is such that its pH is not less than 10.